1. Gas Exchange Partial pressures of gases Composition of lung gases Alveolar ventilation Diffusion Perfusion = blood flow Matching of ventilation to perfusion

2. Partial Pressure of Gas P = nRT V gas equation P gas 1 + P gas 2 + P gas 3……= Total P (e.g. 760 mmHg) partial pressures of gases

4. Gas Transfer

5. . Henry’s law: conc dissolved gas  partial pressure of gas x solubility index Also: Dissolved gas in equilibrium with gas combined with other agents e.g. hemoglobin Gases in liquids diffuse from area of higher partial pressure to area of lower partial pressure.

7. P A > P B O2O2 CO 2 mixed venous blood venous 40 mmHg 100 mmHg P A O 2 100 mmHg 46 mmHg 40 mmHg P A CO 2 40 mmHg Pulmonary vein = arterial

8. Gas diffusion cont. Rate of diffusion pressure difference surface area solubility index   membrane thickness molecular weight  Pressure difference:≈ 60 mmHg for O 2 ≈ 6 mmHg for CO 2 Surface area:large depends on ventilation: perfusion matching Solubility index if:O 2 = 1, CO 2 = 20 Membrane thickness:small

9. Respiratory membrane

10. Fig. is a scanning electron micrograph of mouse lung to show an interalveolar septum. Red blood cells

11. A surface view of capillaries in the alveolar wall. Cross-sectional view of the alveolar wall and their vascular supply.

12. Diffusing capacity: = volume of gas that diffuses through respiratory membrane per minute for each mmHg of gas partial pressure difference  with exercise —  capillary diameter ~ better matching of ventilation to perfusion (blood flow)

14. Alveolar Ventilation Air in alveoli + airways Dead space = air not involved in gas exchange ~ anatomical: air in airways (in ml ≈ body weight in pounds) ~ alveolar: air in alveoli with  blood supply little gas exchange ventilation wasted very small normally V T = V A + V D tidal volume air to alveoli dead space 500 ml = 350 ml + 150 ml

15. Minute/total ventilation = V T  breaths / min Alveolar ventilation = VAVA = (V T  V D )  breaths / min (500  150 )  12 ≈ 4.2 L / min

16. Ventilation : Perfusion Matching Pulmonary circulation flow rate in systemic circulation = flow rate in pulmonary circulation blood pressure in pulmonary circulation mean ≈ 15 mmHg << blood pressure in systemic circulation mean ≈ 100 mmHg

17. Even with exercise Mean P < 30 mmHg

19. Shunts = blood that returns to L side of heart without being oxygenated ~ anatomical: ≈ 2% directly to heart from central chest tissues ~ physiologic / pathologic: blood from areas of lung where blood flow > airflow so some blood flow wasted

20. Factors which tend to disturb ventilation / perfusion ratio: non-uniform pulmonary blood flow non-uniform ventilation pathological gravity effects

22. Effects of Gravity at normal breathing volumes: 1. change in volume at lung bases is greater than at apices i.e. alveolar ventilation > alveolar ventilation (at bases)(at apices) 2. blood flow (perfusion) > blood flow (perfusion) (at bases)(at apices) but Overall Baseperfusion > ventilation Apexventilation > perfusion

26. Protective reflexes which match ventilation with perfusion: 1.Hypoxic pulmonary vasoconstriction – NB if blood flow > airflow 2. PCO 2 control of airway smooth muscle if airflow > blood flow  PO 2 in lung tissue vasoconstriction  blood flow  PCO 2 in lung tissue broncho-constriction  air flow

27. Gas Exchange (summary) Alveolar vs total ventilation Alveolar gas composition (partial pressure) Gas transfer partial pressure of gas in liquid rate of gas diffusion diffusing capacity pulmonary circulation ventilation : perfusion matching dead space shunts protective reflexes